It's an interesting question. Given that in the movie, Obi-Wan first attempted to contact the Jedi Council, and only when he found they were
out of range did he try contacting Anakin, why did he simply record a message and not get R2 to relay his call directly to the Jedi Council?
After all, we see in a later scene that the ship on Tatooine has a good line to the Jedi Council. Could Obi-Wan, one of the wisest Jedi knights
ever, have not thought of this trivially simple expedient?

Oh wait, this is the same Obi-Wan who "hid" Darth Vader's son on his home planet, with his only living relatives. Okay, maybe it all fits
together.

Or was there another reason?

Transcript

Obi-Wan: R2, take a message. I'm on...Obi-Wan: What's the name of this planet?GM: Geonosis.Obi-Wan: ... Geonosis.Padmé: Cool!GM: It is?Padmé: Yeah: geo, nosis. It means "the process of becoming Earth-like". Nice name for a young volcanic world.GM: Right. Totally what I intended.Obi-Wan: Actually, R2, patch me through to the Jedi Council. I need to speak directly to them.Obi-Wan: I'm sure a droid of your "superior capabilities" can manage that.R2-D2: What was that? <krshkhk>R2-D2: ... your signal... <krshkhk>R2-D2: ... going through a tunnel... <krshrshrshkhk>R2-D2: ...aking up... <ksshhhh>